In the urban residential building stock, a major proportion is constituted by low-rise individual buildings. In addition to cost, quality and duration, energy consumed for the project needs to be accounted in the decision making process. Minimizing the cost of construction without compromising on the architectural and structural requirements is the primary objective of the residential buildings of stake-holders, especially the owners. The choice of structural system and the materials used for construction play a crucial role in this effort. This means that the use of expensive and/or voluminous materials such as cement, steel, masonry etc. is optimized. This could lead to significant reduction in embodied energy as well, if the choice of the structural system is prudently made. In this paper, an attempt has been made to quantify the cost and embodied energy benefits for a low-rise residential building by choosing two different structural systems, namely moment resisting framed (MRF) construction system and the partly load-bearing (PLB) system. The influence of choice of materials, contributing to reduction of cost and/or energy is discussed. It is clearly noticed that, when the structural system is re-configured as a PLB system from the existing MRF system there is significant reduction in cost and embodied energy without changing the architectural form.
Building construction involves assembling a wide range of materials from different sources to provide the required structural design, serviceability and aesthetics. During this process the major concern is the project cost and the quality of the materials that go into construction. Along with cost and quality, the duration of the project is also of great importance. With the increase in the affordability among the urbanites, they look for constructing their unique dream houses with their desired specifications. In doing so, the stakeholders tend to oversee if the particular choice considered is the only option available to serve the purpose and at what cost is it being used. This is important because almost all the primary building materials like cement, steel, aggregates, timber etc. are expensive due to their extensive use and shortage in their availability, also some of these materials-cement and steel are energy-intensive. There is a rise in concern to monitor the built-up environment to bring down energy, cost and to educate individuals regarding the alternatives available. Due to this, the concept of “Life cycle energy” of the building is gaining prominence. Along with cost, quality and duration, the energy consumed for the project should also be added as one of the dimensions for the decision making process.
Building consumes energy at every stage of its material production, transportation, execution, maintenance and demolition. It is necessary to take up critical decision making processes during the initial design stage of building so as to reduce the total life cycle energy of the building. Life cycle energy of the building involves the summation of embodied energy, operational energy, maintenance energy and demolition energy. The energy required to initially produce the building is referred as embodied energy. It includes the energy used for the extraction, the processing and the manufacture of the materials of the building as well as their transportation and assembly on site. Operational energy is the energy used to operate the building, in other terms, provide heating, cooling, lighting and power the various appliances of the building. Maintenance of energy is needed to refurbish and maintain the building over its lifetime. The energy is used to demolish and dispose of the building at the end of its life accounts for demolition energy. By carefully assessing the properties of materials for construction, embodied energy of the building can be controlled. Today it is possible to make the building operate at very low operational energy by the use of passive ventilations and from the advent of energy efficient appliances. Use of materials with long life span and those which can be reused and recycled contribute to the reduction in maintenance energy and leads to material conservation [
In addition to life cycle energy of a building, there are also cost components which play a vital role in the decision making processes. This includes capital cost, operational cost, maintenance cost and demolition cost which have to be considered when carrying out the life cycle analysis of building with respect to financial considerations. This paper discusses the relation between embodied energy and initial cost of construction.
It is essential that the initial investments of energy and cost, namely embodied energy and initial cost of construction should be made wisely. Once the building is constructed, it is impossible to reduce these parameters and at further stages any change introduced will contribute to increase in the energy and cost. Therefore by preparing a good initial design plan, carefully selecting the materials and having fewer re-works the buildings’ embodied energy and cost can be optimized [
Globally, owning a house is among the most cherished aspirations of an individual. Due to urbanization, cities are growing ever so rapidly leading to a high demand for land. Vertical growth seems to be a normal solution in solving the housing demands in any city. An on-site walk-down survey conducted for different wards in Bengaluru highlighted the variations observed in residential housing stock for a typical urban Indian city. The buildings can be categorized based on occupancy and height as in
The low-rise individual tenements account to nearly 35% of the building stock as shown in
Structurally low-rise buildings can be configured as Moment Resisting Frames (MRF) or as Load-bearing masonry. The preference for constructing a load- bearing masonry structure is declining of late. Most of the contemporary architects and structural engineers have started designing and configuring low-rise residential buildings as reinforced concrete framed structures. These MRF structures are perceived to offer greater flexibility of design, especially in floor plans. One of the other important reasons for the choice of MRF construction is that
# | Typical urban residential building typology | Type of structural design |
---|---|---|
1 | Single storied individual tenements | Reinforced Concrete/Masonry |
2 | 2 - 3 storied individual tenements | Reinforced Concrete/Masonry |
3 | Multi-Storied apartments (7 - 30 stories) | Reinforced Concrete |
4 | Residential quarters-Group housing (2 - 5 stories) | Reinforced Concrete/Masonry |
5 | Low income group houses (Low Income Group-2 Stories) | Reinforced Concrete/Masonry |
the “open ground storey” can be incorporated. This is almost essential in the urban context, to allow for parking of vehicles. Also, all the interior walls can be made thinner, thereby increasing the “carpet area” On the other hand, and these issues cannot be addressed easily in load-bearing masonry system.
Structurally, load-bearing system is an ideal configuration if the floor plans are identical and the number of stories is limited to say three to four. This is because load-bearing masonry system dominantly develops compressive stress which is of less intensity due to large foot print area of the walls. All the load- bearing walls share the vertical load over their entire length where as in a MRF structure; the vertical loads are concentrated at the location of the columns alone. Also the load-bearing systems are almost devoid of huge bending moment which is very high in MRF system.
It would be an interesting exercise to combine the relative merits of both MRF system and load-bearing system. Indeed such building configurations (termed here as partly load-bearing (PLB) structures) are not uncommon in Bengaluru, especially in relatively small residential plots. The case studies taken up in the present work are one such building which is amenable to be re-configured as PLB system. Of course any change in the structural configuration has an influence on energy and cost.
The objectives of the present work can be listed as follows
・ To compute the embodied energy and initial cost of construction of a typical ground floor plus two storied individual tenement constructed as moment resisting frame (MRF);
・ Re-configure MRF as partly load-bearing (PLB) system and study the variation in embodied energy and cost;
・ Study the influence of alternative structural configurations and alternative masonry on embodied energy and cost of the residential building.
Low-rise structures constitute to a bulk of residential buildings in India. Even in the modern context, a significant share of residential plot is allocated for construction of 2 - 3 storied houses. Many of such houses are constructed keeping in mind the contemporary aspirations of the mid-income-group families. Bengaluru is a typical example of one such city where one can notice a wide spectrum of architectural forms in the low-rise structures, be it in the older wards of the city or in the growing suburban. A typical street as shown in
For the analysis, majority of the embodied energy values of different building materials are considered from on-going research work and published literature pertaining to Bengaluru region [
# | Material | Specification | Embodied Energy | Unit | Cost (INR) | |||
---|---|---|---|---|---|---|---|---|
Unit | Minimum value | Maximum value | Considered value | |||||
1 | Cement | Portland Cement | MJ/m3 | 5184 | 9648 | 5184 | per m3 | 16,800 |
2 | Sand | Natural | MJ/m3 | 153.7 | 175 | 153.7 | per m3 | 1450 |
Manufactured | MJ/m3 | 132.35 | 223.85 | 223.85 | per m3 | 1450 | ||
3 | Gravel | Crushed stone, gravel/chipping | MJ/m3 | 183.25 | 186.29 | 186.29 | per m3 | 900 |
4 | Reinforcement Steel | HYSD Bar | MJ/kg | 24.4 | 42 | 26.84 | per kg | 51 |
5 | Stainless Steel | MJ/kg | 51.5 | 56.7 | 51.5 | per kg | 288 | |
6 | In-fill Masonry | Solid concrete block-400 × 200 × 200 | MJ/block | 5.66 | 15.56 | 7.8 | per block | 41 |
7 | Putty | Lime | MJ/kg | 5.3 | 8.1 | 8.1 | per m3 | 2743 |
8 | Paint | General | MJ/m2 | 20.4 | 29.12 | 25 | ||
Solvent borne | per liter | 198 | ||||||
Antifungal paint | per liter | 294 | ||||||
9 | Wood/ Processed Wood | Frames | MJ/m3 | 870.75 | 870.75 | 870.75 | ||
Doors | MJ/m3 | 5950 | 5950 | 5950 | ||||
Sal Wood | per m3 | 45,000 | ||||||
Teak Wood | per m3 | 123,000 | ||||||
10 | Flooring Material | Polished Granite | MJ/m3 | 436.5 | 436.5 | 436.5 | per m2 | 1800 |
Ceramic Tiles | MJ/m3 | 15,750 | 18,602.5 | 15,750 | per m2 | 450 |
Note: 1USD$ is approx 68 INR (Rs.) and 1EUR$ is approx 71 INR (Rs.) as on 20/12/2016.
The study is taken up to get insights into the energy and cost patterns of a typical building typology which constitutes to nearly 35% of residential housing segment in Bengaluru, India [
A real time case study was considered. It consists of two residences. The ground floor consists of a two bedrooms, living hall and kitchen. The first and second floor adds up to a duplex with four bedrooms, living hall and kitchen. The total built-up area is 280 m2. The study is being carried out to assess if there could have been any reduction in the energy and cost consumption, had alternate structural systems and/or materials been used in this building construction.
The building is constructed as a reinforced concrete moment resisting framed structure with 1:1.5:3 proportioned concrete and high yield strength deformed bars (HYSD). Solid concrete block masonry is used as in-fills. The flooring provided is polished granite in the majority of the area, except in bath where ceramic tiles have been laid. The walls are plastered and painted throughout the building. Teak wood and Sal wood have been used for the frames and shutters of doors and windows. The typical floor plan with column and beam details is as indicated in the
For sample embodied energy and cost calculations refer Annexure A.
From the bill of quantities prepared it was clear that the three parameters considered (volume, energy and cost) are concentrated in super-structure elements of the building. The percentage contributions from sub-structure, super-struc- ture and finishes are indicated in Figures 4(a)-(c) respectively. To achieve a reduction in the respective quantities the first step would be to assess and reduce the super-structure elements which are predominantly made of reinforced concrete.
The quantity of different materials used in the building and their individual embodied energy and cost were computed. The
# | Type | Item | Description | Quantity (m3) | Embodied energy (MJ) | Cost (INR) |
---|---|---|---|---|---|---|
1 | Sub Structure | PCC | 1:4:8 proportion | 15 | 8981 | 35,355 |
2 | RC footing | 1:1.5:3 proportion | 11 | 35,593 | 88,471 | |
3 | RC Pedestal | 1:1.5:3 proportion | 4 | 20,206 | 46,568 | |
4 | Plinth Beam | 1:1.5:3 proportion | 5 | 18,245 | 44,045 | |
5 | Super Structure | RC Beam | 1:1.5:3 proportion | 10 | 70,863 | 152,988 |
6 | RC Column | 1:1.5:3 proportion | 10 | 86,036 | 175,260 | |
7 | RC Roof slab | 1:1.5:3 proportion | 36 | 87,596 | 231,879 | |
8 | RC Staircase & Railings | 1:1.5:3 proportion, Stainless steel | 3 | 39,228 | 170,641 | |
9 | Lintel, Sill & Chejja | 1:1.5:3 proportion | 10 | 24,283 | 62,589 | |
10 | Block Masonry | 150 mm and 10 mm wall | 69 | 55,339 | 248,582 | |
11 | Finishes | Plastering | 1:6 mortar | 17 | 16,222 | 63,378 |
12 | Putty | Lime | 3 | 3772 | 8302 | |
13 | Painting | Solvent borne | 3 | 37,834 | 67,393 | |
14 | Door, Windows | Sal wood & Teak wood frames and doors with steel grills | 6 | 37,956 | 364,623 | |
15 | Flooring and Dado | Polished Granite & Ceramic tiles | 3 | 25,242 | 353,640 | |
TOTAL | 567,396 | 2,113,714 |
Considerable savings in embodied energy and cost can be achieved.
To address the issue of reduction in embodied energy and cost, the building was configured as a partly load-bearing masonry system without compromising any of the functionality or serviceability of the building as offered in the previous MRF system. The configuration is based on designing the wall-above-wall as load-bearing, thereby reducing the number of columns and beams to the
maximum extent possible.
A Brief Note on Structural Configuration of PLB Structural SystemThe walls of a load-bearing masonry building are essentially configured to ensure that the structure is “stable” for all combination of loads. Thus the design approach of masonry structure is based on stability criteria. Masonry structures gain stability from the support offered by cross walls, floor/roof diaphragm and other elements such as piers. There are extensive guidelines available to ensure the stable configuration [
It is rather important to understand the computation of sharing of loads by masonry walls. This can be explained by a simple example of a rectangular building with four walls orthogonal to each other forming a box-type configuration. While computing the load on a pair of parallel walls it is assumed that the other two walls are redundant. Thus the entire load is deemed to be shared by the two walls. Therefore, ideally load-bearing masonry buildings should have a good disposition of walls along both the horizontal directions. This, not only helps in ensuring stability, but also in transferring the lateral loads (mainly wind and seismic loads) in proportion to their in-plane stiffness.
The load-bearing walls of the partly load-bearing system can also be designed by the above stated principles. Of course the location of the columns and beams dictate the tributary area and the loads shared thereof. It is important to note that the moments are not transferred to the un-reinforced walls at the wall-beam junction. The walls of the case study taken up are thus designed.
The load-bearing design for a typical floor is as shown in
masonry foundation is considered and the external walls in the rear portion of the building are designed as load-bearing. Few columns could not be dispensed with, for this building, since there was a need to have a wall-free space in the car-parking space and portico in the front portion of the building. Major beams are also retained for long span slabs. Due to these changes there is a change in the slab detailing. All the finishing elements are retained as per the initial design with solid concrete blocks as the masonry element. The
# | Type | Item | Description | Quantity (m3) | Embodied energy (MJ) | Cost (INR) |
---|---|---|---|---|---|---|
1 | Sub Structure | PCC | 1:4:8 proportion | 3 | 1922 | 7857 |
2 | SSM footing | 7 courses | 27 | 16,554 | 33,044 | |
3 | Plinth Beam | 1:1.5:3 proportion | 3 | 7516 | 19,215 | |
4 | Super Structure | RC Beam | 1:1.5:3 proportion | 8 | 45,280 | 100,956 |
5 | RC Column | 1:1.5:3 proportion | 4 | 19,518 | 42,472 | |
6 | RC Roof slab | 1:1.5:3 proportion | 29 | 65,019 | 177,314 | |
7 | RC Staircase & Railings | 1:1.5:3 proportion, Stainless steel | 3 | 39,163 | 170,643 | |
8 | Lintel, Sill & Chejja | 1:1.5:3 proportion | 10 | 24,204 | 62,589 | |
9 | Block Masonry | 200 mm & 100 mm wall | 89 | 59,811 | 272,296 | |
10 | Finishes | Plastering | 1:6 mortar | 17 | 15,176 | 63,378 |
11 | Putty | Lime | 3 | 3772 | 8302 | |
12 | Painting | Solvent borne | 3 | 37,834 | 67,393 | |
13 | Door, Windows with grill | Sal wood & Teak wood | 6 | 37,956 | 364,623 | |
14 | Flooring and Dado | Polished Granite & Ceramic tiles | 3 | 22,836 | 434,497 | |
TOTAL | 396,562 | 1,824,579 |
PLB system as compared to the MRF there by offering a significant reduction in the embodied energy and cost. But the volume of masonry used has increased by the introduction of load-bearing walls. The higher proportion of energy and cost due to increased quantity of masonry can be brought down by choosing a more energy efficient and cost effective masonry unit as compared to solid concrete blocks.
The overall values of embodied energy, cost and quantity of steel consumed in the two designs under consideration have been graphically represented in
Considering that the major stake-holders (owner, architect and engineer) wish to retain the structural system of the building as MRF but still need a reduction in the embodied energy and cost, then it is the choice of masonry units which needs to be reviewed. The masonry elements’ contribution to percentages of volume, energy and cost parameters of the MRF building design is about 33%, 10% and 12% respectively when solid concrete blocks (SCB) are used.
A range of acceptable masonry units as listed in
# | Type | Block size (mm) | Embodied Energy (MJ/block) | Cost (INR/ unit) | ||
---|---|---|---|---|---|---|
Minimum value | Maximum value | Considered value | ||||
1 | Solid block concrete | 400 × 200 × 200 | 5.66 | 15.56 | 7.8 | 41 |
2 | Hollow Concrete blocks-10% cement | 400 × 200 × 200 | 7.6 | 12.3 | 7.6 | 40 |
3 | Table-moulded Brick | 230 × 105 × 75 | 3.28 | 7.15 | 7.15 | 6.5 |
4 | Stabilised Mud Blocks 8% cement | 230 × 190 × 100 | 2.85 | 3.5 | 2.85 | 25 |
5 | Autoclaved aerated concrete Block | 600 × 200 × 200 | Range of values not available | 58.8 | 120 | |
6 | Fly Ash Brick | 225 × 100 × 75 | 5.3 | 5.5 | ||
7 | Hollow clay Block | 400 × 200 × 200 | 16.7 | 47 |
embodied energy value variations for the in-fills, all the minimum and maximum values were considered and the analysis was carried out. The variation is graphically represented as in
The embodied energy of buildings constructed with stabilized mud blocks (SMB) is lower than the other alternatives. This is due to the non-requirement of any burning energy during their production and also due to the fact that very little cement is required in the preparation. Also, SMB walls do not need plastering and painting due to their aesthetic appearance, thereby saving a lot of cost and energy. The contribution of embodied energy from plastering and painting is around 3% and 7% of the total embodied energy, respectively. Thus a considerable saving can be achieved by avoiding these finishing items. Also, by replacement
of existing SCB by low embodied energy SMB, up to 9% saving in energy can be achieved.
Engineered Hollow concrete blocks (EHCB) are also energy efficient as there is reduction in volume of material consumed for the same strength characteristics when compared to a SCB. Though SMB units are less expensive when compared to EHCB, their overall cost increases due to high mortar requirement. With EHCB as replacement, percentage saving in embodied energy and cost will be of the order 3.5% and 3.2% respectively.
Table-molded bricks (TMB) are known to be one of the most energy intensive masonry units, due to inefficient burning process at a high temperature of 8000Cduring their production. If SCB are replaced by TMB, embodied energy of the building increases by 42%. With respect to cost, the autoclaved aerated concrete (AAC) blocks are the most expensive as they involve highly automated production process. They are often transported over long distances, for instance there is no AAC block manufacturing plant near Bengaluru, they are transported from Hyderabad or Chennai. This adds to the transportation cost and hence the use of AAC blocks lead to 7% increase in cost. Thus it can be concluded that the choice of masonry element to be considered for the building will play a crucial role.
Considering the above alternatives it is apparent that MRF buildings in combination with EHCB in-fills, lead to reduction in embodied energy and construction cost significantly.
It is already noticed that by a shift from MRF to PLB design there is a substantial saving in energy and cost. It would be a useful exercise to bring down the cost and energy further, by considering the alternative load-bearing masonry elements listed in
In PLB system, replacing SMB by SCB leads to 12% saving in energy, while, with EHCB as replacement, results in 5% and 4% savings in embodied energy and cost respectively. Thus the saving in energy and cost achieved through shift from MRF to PLB system can be further enhanced by using EHCB.
For the range of masonry units acceptable in an urban scenario the embodied energy of the case study varies from 1.25 GJ/m2 to 2.89 GJ/m2 depending on the choice of structural system. The cost ranges from a minimum of Rs. 6258/m2 to Rs. 8090/m2.
It can be noted that there exists a range of values for both the structural systems. The overlapping of embodied energies values in the two structural systems is evident; however the cost of all PLB system falls lower than those of MRF system with alternate in-fills. This indicates that there is definite saving in cost that can be achieved from switching over to PLB system with any choice of alternate in-fills.
In the present study a two storied residential building which is one among the
# | Masonry Type | Embodied Energy (GJ/m2) | |
---|---|---|---|
MRF | PLB | ||
1 | Autoclaved aerated concrete Blocks | 2.76 | 2.21 |
2 | Solid block concrete | 2.03 | 1.42 |
3 | Engineered Hollow Concrete blocks-10% cement | 1.96 | 1.35 |
4 | Table-moulded Bricks | 2.89 | 2.38 |
5 | Fly Ash Bricks | 2.66 | 2.12 |
6 | Stabilised Mud Blocks 8% cement | 1.85 | 1.25 |
7 | Hollow clay block | 2.22 | 1.64 |
# | Masonry Type | Cost (INR/m2) | |
---|---|---|---|
MRF | PLB | ||
1 | Autoclaved aerated concrete Blocks | 8090 | 7106 |
2 | Solid block concrete | 7549 | 6516 |
3 | Engineered Hollow Concrete blocks-10% cement | 7306 | 6258 |
4 | Table-moulded Bricks | 7822 | 6817 |
5 | Fly Ash Bricks | 7704 | 6688 |
6 | Stabilised Mud Blocks 8% cement | 7882 | 6931 |
7 | Hollow clay block | 7586 | 6550 |
typical representatives of urban residential typology has been considered. The following set of broad conclusions can be drawn:
・ Buildings re-configured as PLB system from the existing MRF system lead to reduction in embodied energy and cost [
・ Major share of reduction in EE and cost is from the reduction in steel and concrete consumption both of which are energy and cost intensive materials.
・ Amongst the various alternatives for masonry it is noted that SMB (with 8% cement) followed by EHCB (with 10% cement) are the two best alternatives with respect to EE.
・ From the point of view of cost it can be concluded that EHCB (with 10% cement) is the most economical masonry option.
・ Masonry alternatives with AAC blocks and WCB are not only expensive but also lead to high embodied energy.
In the present study, the two major alternatives suggested, namely PLB system in place of MRF and a variety of choices for masonry have indeed been adopted, indicating its acceptance. One can come across quite a good number of individual homes in Bengaluru with PLB systems. However, it is hard to readily recognize this system once the building gets the finishing elements.
amongst the peer group of the society. There is a need to promote such cost and energy effective alternatives. Any discussion related to suggesting alternatives to residential buildings, especially to the stakeholders of individual houses would be found lacking if the issue related to social acceptance is not brought in.
The work reported in this paper is supported by BMS College of Engineering, Bengaluru, through Technical Education Quality Improvement Programme [TEQIP-II] of the Ministry of Human Resource Development, Government of India.
Varsha, B.N., Raghunath, S. and Keshava, M. (2017) Influence of Choice of Structural System & In-Fill Masonry on the Embodied Energy & Cost of a Low-Rise Residential Urban- Building Indian Case Study Open Journal of Energy Efficiency, 6, 41-60. https://doi.org/10.4236/ojee.2017.61003
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